A great many machine tools, and particularly more sophisticated machine tools, are driven by servo-motors under the control of an optical tracer, a mechanical tracer or a tape readable control. To obtain accuracy in the machining operation it is important to reduce the amount of backlash in the drive between the servo-motor and the movable machine member and keep it to an absolute minimum. Many machine tools employ a rack and pinion drive where the pinion is carried by the output shaft of a gearbox, and in such tools the greatest contribution to the total backlash is caused by the free-play between the rack and the pinion. In smaller machine tools such as flame cutting tools that do not have a high order of precision compared to metal working tools such as milling and boring machines, the machine ways or rails are frequently not in precision alignment and some means must be provided to accommodate the resulting misalignment produced between the rack and the pinion.
In order to eliminate the misalignment between the rack and pinion gears, the servo-drives are frequently cantilevered on a pivoting member, which allows the pinion to be pressed against the rack with the aid of a coil compression spring. With this arrangement the backlash between the pinion and the rack is minimized since the pinion can follow small variations in clearance due to rack and pinion inaccuracies. This pivoting servo-support has proved satisfactory in reducing misalignment between the drive pinion gear and the rack gear, but the cantilevered support has created a problem in adequately supporting the pinion gear and also in adequately supporting the entire drive unit for pivotal movement because the drive unit usually has considerable weight. Another disadvantage, and not an insignificant one, is that the pivotal servo-gearbox is quite expensive since it employs pre-loaded roller bearings to support the cantilevered weight.
A pivotally mounted drive unit similar to that described above is shown in the Brouwer U.S. Pat. No. 4,072,301. In this prior assembly, the drive unit has an overhanging bottom plate through which the output drive pinion projects and an overhanging portion of the plate extends under a specially designed slide frame plate and is pivotally supported on the plate by a pivot bolt with thrust bearings between the top and bottom plate of the drive unit and the special frame plate and another thrust bearing on top of the frame plate.
Another pivotal mount arrangement for a flame cutting machine gearbox assembly is shown in the Dobrauz U. S. Pat. No. 3,552,224. The Dobrauz gearbox assembly is a pivotal mounting consisting of widely spaced bearings and a fixed yoke member adapted to be connected to one of the slide frame parts. A pivot shaft is supported in these bearings and connected at both projecting ends of a yoke on the gearbox housing. While this arrangement has been satisfactory in many applications and provides adequate support for the gearbox housing assembly, it is costly and the yoke on the gearbox require that the gearbox output shaft be extended downwardly away from the gearbox a considerable distance to mate with the fixed rack because of part interference. This results in an undesirable cantilevering of the output pinion gear that reduces the accuracy of the drive.
Other examples of pivotally mounted gearboxes are found in the following patents: Rieser U.S. Pat. No. 2,838,229, Wall U.S. Pat. No. 3,469,068, Caldwell U.S. Pat. No. 3,739,652, Dreshman U.S. Pat. No. 3,866,484, and Kensrue U.S. Pat. No. 4,158,315.
It is a primary object of the present invention to ameliorate the problems noted above in drive unit assemblies for pattern or tape controlled machinery.